lavf: check that the context to avformat_open_input() is valid.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -589,6 +589,10 @@ int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputForma
 
     if (!s && !(s = avformat_alloc_context()))
         return AVERROR(ENOMEM);
+    if (!s->av_class){
+        av_log(0, AV_LOG_ERROR, "Input context has not been properly allocated by avformat_alloc_context() and is not NULL either\n");
+        return AVERROR(EINVAL);
+    }
     if (fmt)
         s->iformat = fmt;